If you applied my code correctly (and changed X too, the URL
http://www.example.com/forums/index.php?mystyle=3
will work provided that you dont apply a mandatory style in your forum.
Say if you have style A, B and C and all of them selectable in all the boards (you can have a default one but not a mandatory one) then my code will change the style of the user if it's called from the address bar. I tested it and it is working..